Research Highlights.

Abstract

Metabolic Reprogramming by Immune-responsive Gene 1 Upregulation Improves Donor Heart Preservation and Function Lei I, Huang W, Noly PE, et al. Sci Transl Med. 2023;15:eade3782. Ex vivo normothermic perfusion platforms are increasingly used in solid organ transplantation.1-3 In heart transplantation, these platforms lead to reduced damages from cold ischemia time and increased donor heart utilization due to minimized geographical limitations.4 Despite this, primary graft dysfunction remains prevalent following machine perfusion, driven by ischemia–reperfusion injury. However, effective treatments are lacking to protect and alleviate damages from cellular ischemia. Lei et al5 evaluated whether valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, could increase the ischemic tolerance of donor hearts. The authors first showed that treatment of VPA maintained histone acetylation following prolonged cardiac preservation of human hearts, confirming efficacy in cardiac tissues. The addition of VPA to the preservation solution of murine hearts improved contractility and relaxation for up to 16 h. This effect was also seen with other HDAC inhibitors, suggesting that the cardioprotective effect was not due to an individual drug but rather to HDAC inhibition. In a syngeneic murine heart transplant model, VPA added to the preservation solution resulted in the improved mechanical function of donor hearts, reduced inflammation, and decreased myocardial damage and cell death following cold preservation. Mechanistically, several pathways were responsible for the salutary effect of VPA on organ preservation. First, VPA treatment reduced succinate production, a precursor of ROS and inflammation, and increased itaconate production, an anti-inflammatory metabolite. Accordingly, VPA induced the expression of IRG1, the gene that encodes the enzyme that synthesizes itaconate. Murine hearts lacking IRG1 had worse function than wild-type hearts without VPA treatment. Furthermore, the deficiency of IRG1 reduced the cardioprotective effect of VPA, both in ex vivo perfusion and syngeneic transplant models. Second, VPA treatment reduced oxidative stress through increased antioxidants via activation of the NRF2 pathway. Finally, the authors assessed the translatability of their therapy in a porcine model. They found that VPA improved the mechanical function and oxygen consumption of pig hearts at both 4- and 10-h preservation time points. Similar to their findings in mice, VPA treatment led to increased expression of IRG1, activation of the NRF2 pathway, and reduced oxidative DNA damage. Despite advances in organ preservation, ischemic injury remains unavoidable in heart transplantation. To date, however, there are no targeted molecular therapies available to improve donor heart preservation and performance after reperfusion. The authors used VPA, an FDA-approved drug, to improve donor heart ischemic tolerance and achieve improved donor heart function. They found that VPA exerted its effect through the activation of several cardioprotective metabolic pathways. This study raises several important questions that are worth investigating in the future: (1) whether a similar strategy (ie, HDAC inhibitors) would also be efficacious in the preservation of other solid organs (eg, kidney, liver, lungs); (2) could HDAC inhibitors be rationally designed to selectively exploit their metabolomic properties; and (3) whether HDAC inhibitors could also be applied to other ischemic–reperfusion pathologies (eg, myocardial infarctions, strokes). This study has clear and exciting implications for the field of solid organ transplantation. HDAC inhibitor treatment has the potential to protect donor organs from ischemic injuries, thus optimizing organ outcomes and utilization. Furthermore, these therapies could have a much broader use with other pathologies related to ischemia. Neutrophil CEACAM1 Determines Susceptibility to NETosis by Regulating the S1PR2/S1PR3 Axis in Liver Transplantation Hirao H, Kojima H, Dery KJ, et al. J Clin Invest. 2023;133(3):e162940. Ischemia–reperfusion injury (IRI) in orthotopic liver transplantation (OLT) is a major cause of short-term liver allograft dysfunction and long-term allograft failure. However, a clear molecular understanding of the cell signaling promoting IRI-promoted sterile inflammation in OLT is lacking, although the broad innate immunity is commonly accepted as a dominant contributor. Within innate immune cell populations, neutrophils have been identified as the primary instigator of liver allograft IRI. Neutrophils, when activated, are known to release “neutrophil extracellular traps” (NETs) that are web-like chromatin structures evolutionarily emerged to trap and kill extracellular microbes via a cellular process called NETosis.1 In OLT-induced IRI, NETs form in response to released endogenous danger signals such as HMGB1 and histones,2 initiating sterile inflammation in the absence of microbes. In this process, neutrophils themselves may utilize either a cell death-independent pathway or a cell death-dependent pathway, resulting in their own demise via autophagy. In the current study,3 Hirao et al examined the role of neutrophil CEACAM1 signaling in liver allograft IRI. CEACAM1, or carcinoembryonic antigen-related cell adhesion molecule 1, is a transmembrane glycoprotein expressed on several cellular populations including neutrophils. Its gene, CC1, can be alternatively spliced to generate functionally distinct short (CC1-S) and long (CC1-L) isoforms. Using a murine genetic knockout model, the investigators first observed that following OLT-induced IRI, recipient neutrophils were the primary source of CC1-L. The absence of CC1-L resulted in exacerbated hepatic IRI. Consequently, CC1-null neutrophils were susceptible to NET formation. This propensity appeared to be mediated by dysregulated downstream S1P receptor pathways, namely, the S1P receptor 2 (S1PR2) versus receptor 3 (S1PR3) preferences. Specifically, S1PR2 expression was significantly upregulated, whereas S1PR3 was significantly suppressed in CC1-null neutrophils compared with CC1-sufficient neutrophils. Interestingly, NETosis regulated by the CC1-S1PR2/3 axis was closely related to regulated neutrophil autophagy. Furthermore, exacerbated NETosis in CC1-null OLT recipients was not limited to the IRI-experienced liver allograft but was also observed in distal native organs such as the lungs, suggesting a systemic effect of neutrophil activation. To demonstrate the finding’s clinical relevance, the investigators retrospectively analyzed 55 human liver biopsies collected at 2 h postreperfusion. They found that those enriched for neutrophil CC1-L showed relative resistance to IRI stress and hepatocellular damage and experienced improved liver allograft function and superior clinical outcomes. This study reveals a novel neutrophil checkpoint regulator, namely, CC1-L, in neutrophil autophagy and NETosis, and their contribution to liver allograft IRI in OLT. These findings not only present the potential of using CC1-L as a biomarker for IRI in the postoperative management of OLT recipients but also provide the basis for identifying potential therapeutic targets to mitigate OLT-induced neutrophil autophagy and NETosis and to mitigate liver allograft IRI. It would be of further interest to examine the involvement of this checkpoint regulator in other types of transplantation, particularly in allogeneic or xenogeneic islet transplantation, which uses the intraportal route for islet engraftment. It is conceivable that the CC1-S1PR2/3 axis could be similarly exploited to minimize the instant blood-mediated inflammatory reaction seen with intraportal islet transplantation.